Aluminum reduction is a process that is difficult to completely automate. There are various reasons for this difficulty. One reason is that the materials used in the process cannot be easily handled with conventional materials handling equipment. Another reason is that it is difficult or impossible to monitor many important variables within the process. As a result, many process conditions must be inferred from a few variables that can be measured.
By way of background, FIG. 1 shows a simplified cross-section of an aluminum reduction cell 20. The reduction cell 20 includes a large thermally insulated container or "pot" 22, insulated with an electrically conductive refractory lining, which forms the cathode 24. Within the pot is a molten mixture 26 of cryolite in which the alumina (aluminum oxide) is dissolved. This liquid layer 26 is commonly referred to as "bath." Sacrificial carbon anodes 28 (one shown) extend into this mixture 26 from above. Pot 22 and a metal pad 30 of molten aluminum form an associated cathode.
A voltage potential is applied between anodes 28 and pot 22, resulting in a large current flow between them and through the bath 26. The magnitude of this current is typically over 60,000 Amperes. The electrical current passing through the alumina mixture 26 converts the alumina into its aluminum and oxygen components by electrolysis. The aluminum drops to the bottom of pot 22, forming the metal pad 30, while the oxygen combines with carbon from anodes 28 and escapes as carbon dioxide gas. As alumina is consumed, more alumina is added to the cell. The carbon dioxide gas is vented away by an overlying hood (not shown).
In order to regulate the aluminum reduction process, it is necessary to control the electrical power in the reduction cell. Typically, electrical current is applied to a "line" of multiple cells arranged in series. Hence, the electrical current passing through an individual cell is a variable that cannot be affected or changed at the cell level. Instead, varying the voltage across the cell regulates electrical power to individual cells. This is accomplished by raising and lowering the anodes relative to the underlying cathode. Raising the anodes increases the distance between the anodes and the cathode, thereby increasing impedance. Since current through the cell is nominally constant, the increased impedance raises the voltage between the anode and the cathode, and thereby increases the overall power in the reduction cell.
During the aluminum reduction process, it is necessary to periodically add (i.e., "feed") alumina into the pot to replace the alumina that has been converted to aluminum. It is important to feed alumina at a proper rate. Too much alumina drastically reduces efficiency. Too little alumina can cause increased impedance, and thus voltage, to rise, eventually causing what is known in the aluminum industry as an "anode effect."
An anode effect occurs when the pot is underfed to the point where the conversion process begins generating fluorocarbon gas bubbles rather than carbon dioxide. These bubbles form a layer of gas of very low electrical conductivity on the bottom of the anodes that increases impedance and thus, voltage and power. The higher power heats up the pot and accelerates the fluorine conversion, thus generating even more fluorine gas and even higher voltage. In a very short time, on the order of milliseconds, this behavior can increase the cell voltage to several times its normal value.
The anode effect can also be explained at the molecular level in terms of ion populations at the surface of the carbon anode. During the aluminum reduction process, oxygen-bearing ions consisting of aluminum, oxygen, and fluorine exist at the anode surface. These anode ions, or "anions", react with the carbon and are consumed. Carbon dioxide and some carbon monoxide are produced as a result. In a normal process, the anions are replaced and the carbon dioxide gas escapes as small bubbles. Replacement anions are transported to the anode surface by convection and diffusion in balance with the electrolytic consumption. The anion transport rates are finite, affected by such phenomena as the electrolyte temperature, the gradient of the electrolyte temperature near the anodes, convective mixing induced by magnetohydrodynamics, and concentration gradients of the relevant ions.
The anode effect occurs when the anion consumption rate exceeds the cumulative transport rates and thus the anions are not replaced fast enough at the anode to continue steady-state operation. The remaining anions are reduced by a reaction of their fluorine component with the carbon, producing a fluorocarbon gas. It is the physical properties of this gas that cause the operational difficulties associated with the anode effect. Specifically, this fluorocarbon gas wets the anode surface, resulting in larger bubbles than those formed by the carbon dioxide. Since the fluorocarbon gas is also dielectric, the larger bubbles effectively insulate the covered portion of the anode surface, so that the covered portion is not electrolytically active. It is noted that the transition from steady-state electrolysis to the anion depletion near the anode that produces the anode effect occurs gradually due to the finite rates of the processes; once the production of fluorocarbon gas begins, the rise in cell impedance occurs rapidly.
While it is considered normal for a pot to occasionally exhibit anode effects, it is nonetheless desirable to reduce the number of anode effects and also to reduce the magnitude of voltage increase during any single anode effect. Reducing anode effects improves operating efficiency and minimizes the quantity of fluorocarbons generated.
There are different strategies for controlling and terminating anode effects. A primary strategy is to accelerate alumina feedings. Another strategy is to rapidly lower the anodes to dispel the gas bubbles, and then raise the anodes back to their proper level. In extreme cases, gas bubbles are dispelled by shoving "green" tree branches beneath the anodes.
Ideally, it would be desirable to predict when an anode effect is about to occur. Unfortunately, this simple premise has heretofore been impossible to implement. One early attempt to predict impending anode effects was based on movement of cell impedance. When the cell impedance increased by a certain percentage, the alumina concentration was known to have decreased and hence the technique assumed that the cell was on the way to experiencing an anode effect. The operator would then try to accelerate the alumina feed to prevent the anode effect. However, this technique proved not to be a very effective prediction tool, especially in high current density cells, and thus it is not in use today.
Accordingly, there remains a need for a system and method that more accurately predicts impending anode effects.